The corrosion of iron and steel as well as concrete and limestone is a wide-spread problem. The cost to governments as well as private industry to replace or repair pipelines, storage tanks, pumps, and other systems that have been damaged by corrosion is substantial. Most corrosion is caused by microbial processes, whether directly or indirectly. Corrosion in general and microbial corrosion in particular are complex processes and seldom, if ever, involve a single mechanism or a single microbial species.
Corrosion may be defined as the destructive attack on metals by some chemical or electrochemical mechanism. In aqueous solution, or in humid environments, bulk metals (M) tend to ionize giving up electrons (e.sup.-) (Equation 1): EQU M.fwdarw.M.sup.n +ne.sup.- ( 1)
The area on the metal surface where this reaction takes place is called the anode. Corrosion occurs when electrons are removed from the metal increasing the net ionization of the metal. The two principal mechanisms for the removal of electrons are an excess of hydrogen ions (H.sup.+) (Equation 2): EQU 2H.sup.+ +2e.sup.- .fwdarw.2H.fwdarw.H.sub.2 ( 2)
or oxygen (Equation 3): EQU H.sub.2 O+1/20.sub.2 +2e.sup.-.fwdarw. 2 (OH).sup.- ( 3)
Areas on the metal surface where these reactions occur are referred to as cathodic areas. In order for significant corrosion to occur, the anodic and cathodic reactions must remain in balance and the electrolytic cell must continue functioning over an extended period of time. In microbial corrosion, the corrosive effects are thought to be the result of one or more of the following mechanisms:
(1) direct chemical action of metabolic products such as sulfuric acid, inorganic or organic sulfides, and chelating agents such as organic acids;
(2) cathodic depolarization associated with anaerobic growth;
(3) changes in oxygen potential, salt concentration, pH, etc., which establish local electrochemical cells; and
(4) removal of corrosion inhibitors (oxidation of nitrite or amines) or protective coatings (bitumen on buried pipes).
Several microbial species may be involved in these processes, either alone or as components of consortia.
In an anaerobic environment, corrosion is most commonly attributed to the growth of dissimilatory sulfate-reducing bacteria (SRB). This group of bacteria is responsible for possibly 50% of all instances of corrosion. Corrosion of steel caused by sulfate-reducing bacteria is characterized by pitting of the metal, with the pits being open and filled with soft black corrosion products in the form of iron sulfides. When the iron sulfide is removed, the metal underneath is bright. Pitting corrosion in aluminum and copper alloys have also been reported. With cast iron, graphitization occurs. The iron is dissolved away leaving the graphite skeleton of the pipe apparently unaffected. Twenty-five years ago, the underground corrosion of iron or steel gas or water pipes cost about one-half to two billion dollars per year. Today, these costs are significantly greater.
As noted above, when oxygen is not available for the removal of electrons from the metal surface, an alternative cathodic reaction is necessary for corrosion to occur. In 1934, von Wolyogen Kuhr and van der Vlugt (Water, 18. 147) suggested that sulfate-reducing bacteria contribute directly to the corrosion of iron by the removal and utilization of hydrogen available at the cathodic areas of the metal for the reduction of sulfate to sulfide. They proposed the following mechanism of cathodic depolarization: ##STR1## The production of inorganic sulfide in the cathodic depolarization reaction is itself very important to the problem of microbially induced corrosion. Sulfate reducing bacteria are the major source of hydrogen sulfide in the biosphere. Hydrogen sulfide is highly toxic and quite corrosive chemically in an aqueous environment. Since this theory of cathodic depolarization was proposed, most of the literature on microbial corrosion has been concerned with testing its validity. For example see: Booth & Tiller, Trans. Faraday Soc., 56, 1689 (1960) and 58. 2510 (1962); Booth et al. Congr. Intern. Corros. Marine Salissures, 363, CREO Paris (1964); Booth et al. Chem. Ind., 49. 2084 (1967); King & Miller, Nature, 233. 491 (1971); Iverson, Nature, 217. 1265 (1968); Iverson, Underground Corrosion (Ed. Escalante), p. 33-52, Tech. Pub. 741, ASTM (a981).
The sulfate reducing bacteria are a taxonomically diverse group of bacteria. In the mid-sixties, only two genera were identified, Desulfovibrio and Desulfotomaculum. The genus Desulfovibrio is the most studied of the genera. The Desulfovibrio species are usually mesophilic, relatively easy to isolate and they do not form spores. Most species of Desulfovibrio contain c-type cytochromes and the chromatophore desulfoviridin. Desulfotomaculum species are either mesophilic or thermophilic. The common thermophilic species, Desulfotomaculum nigrificans was originally called Clostridium nigrificans. The formation of spores is restricted to and a characteristic of the genus Desulfotomaculum. Widdel and Pfenning (Arch. Microbiol., 131. 360, 1982) recently described five new genera of sulfate-reducers (Desulfobacter. Desulfobulbus. Desulfococcus. Desulfonema and Desulfosarcina) which are not only morphologically distinct, but also nutritionally diverse.
The metabolic capabilities of the dissimilatory sulfate-reducing bacteria in the presence of sulfate show great variation both among genera, and within certain genera, such as Desulfovibrio. Sulfate-reducers have been shown to use a wide range of carbon compounds as electron donor including alcohols, organic acids such as lactate, pyruvate and benzoate, and fatty acids from formate to stearate. Many of the Desulfovibrio species such as D. desulfuricans. D. gigas. and D. sapovorans, as well as Desulfotomaculum nigrificans degrade lactate and pyruvate to acetate and reduce sulfate to sulfide. Other species completely oxidize long-chain fatty acids as well as aromatic acids to CO.sub.2. Hydrogen also serves as the electron donor for many species and these bacteria can participate in interspecies hydrogen transfer reactions by utilizing the H.sub.2 produced by the fermentative bacteria. In addition to utilizing H.sub.2, some sulfate reducers can also produce H.sub.2 from organic molecules.
The sulfate-reducing bacteria can be found in muds, ponds, sewage, fresh and marine waters, underground aquifers, oil reservoirs, as well as the rumina of sheep and cattle and the guts of insects. These organisms flourish in polluted lakes and canals. Thermophillic sulfate-reducers are usually strains of the species Desulfotomaculum nigrificans and can be found in deep telluric aquifers subject to geothermal heating. Sulfate-reducers can also grow in the aqueous phase of oil and petroleum storage systems. Since many oil reservoirs have high concentrations of sulfate, sulfide production by sulfate-reducing bacteria is a major economic concern to this industry. Sulfate-reducing bacteria appear to be indigenous in oil reservoirs, although this is still being debated (see Nazina, Geomicrobiol, J., 4, 103, 1985). These organisms are readily introduced into wells during secondary oil recovery by water flooding and have been reported to penetrate between 0.6 and 2 m per year through oil-bearing sands.
Today, most experimental evidence indicates that sulfate-reducers do not oxidize hydrocarbons, but the question has not been completely settled. Recent studies have shown that anaerobic methane oxidation occurs at depths where rapid sulfate reduction occurs suggesting that these two processes are linked.
The growth of sulfate reducing bacteria in oil field water systems can be costly to the industry. It has been well established that the growth of sulfate-reducing bacteria can promote corrosion of pipelines, well casings, storage tanks, pumps, etc., plug injection wells and possibly degrade polymers (polyacrylamide and xanthum gums) and surfactants. Almost every aspect of oil recovery is affected.
Although the first study of microbially induced corrosion in pipes dates to circa 1923, the role of microorganisms as a major source of corrosion in the oil field is still not appreciated in certain quarters. The skeptic should consider the following case history. A major oil company operating an offshore platform in the far east laid a 60 mile, 23" diameter, subsea pipeline to carry production to onshore storage facilities. The crude oil contained less than 1% water. The production rate was such that laminar flow existed in the pipe and the entrained water settled to the bottom of the pipe. Unknown to the field engineers, significant microbial growth developed in the water layer and in time leaks were detected in the pipeline. Examination of the pipeline revealed holes in the underside of the pipe every 3-4 ft. Virtually, all of the pipeline had to be replaced after only four years of operation. When the pipeline was put back in operation, a program for monitoring and control of microbial growth was initiated. The new pipeline has been in operation for ten years without any indications of corrosion.
There are numerous other examples of multi-million dollar lessons in microbial corrosion, formation plugging and failure of enhanced oil recovery programs due to ineffectiveness of mobility control agents. It is impossible to operate an oil field water system under sterile conditions. However, the above case history clearly illustrates the need to minimize microbial growth in these systems.
The control of microbial corrosion in oil recovery operations generally incorporates both physical or mechanical treatment and chemical treatment. Physical methods include:
(1) choosing injection water sources to minimize sulfate-reducing bacteria inoculation;
(2) periodically pigging or scraping water lines and flushing with slugs of surfactants and solvents;
(3) avoiding commingling of waters from different course since commingling can improve the growth environment of sulfate-reducing bacteria; and
(4) eliminating dead spots and reducing water handling time.
Although physical methods can made a significant contribution to the control of sulfate-reducing bacteria, the most effective control of microbial activity in an oil field water system is obtained by chemical (biocide) treatment. A number of factors are considered when choosing a biocide or biocides for an individual treatment situation. First, the biocide(s) of choice must be active against the bacteria in the water system under the conditions which exist in the system and preferably in the reservoir as well. Secondly, the biocide must be persistent in the system; that is, capable of reaching points far removed from the source while retaining activity. The injection water represents a complex chemical environment. The biocide must exhibit chemical stability in this environment. Further, the biocide must be compatible with chemical treating agents such as corrosion inhibitors, scale inhibitors, oxygen scavengers, etc. Lastly, the biocide must be economical.
Organic biocides, such as glutaraldehyde, generally offer a high degree of persistence in an oil field water system. However, the effectiveness of any organic biocide is dependent upon the water chemistry and microbiology of the system in which it is applied. A biocide may work very well in controlling sulfate-reducing bacteria in one field but be totally ineffective in another.
Sulfate-reducers are sessile bacteria; that is, they tend to attach themselves to a solid surface. In an oil field water system, sulfate-reducers are generally found in combination with slime forming bacteria in films composed of a biopolymer matrix embedded with bacteria. The interior of these films is anaerobic and highly conducive to the growth of sulfate reducing bacteria even if the surrounding environment is aerobic. If a biocide is to be effective against sessible bacteria, penetration and absorption by the biofilm which protects these bacteria is required. Therefore, much higher concentrations of costly biocide are required to control bacteria embedded in a biofilm than planktonic or free-floating cells of the same species.
The physical and chemical control of the growth of sulfate-reducing bacteria represents a significant and on-going expense in any oil field operation as well as in other operations where the production of H.sub.2 S is a problem. More effective methods of controlling sulfate-reducing bacteria and biogenic hydrogen sulfide production and accumulation are needed.